BigQuery: Add enum with allowed data types for use in Schema

[tswast]

I received a feature request internally for an enum with allowed data types for use in SchemaField.

The string values should match the names of the BigQuery legacy SQL data types, but we could potentially add additional constants using the standard SQL data type names, but still the legacy SQL string value.

[tswast]

We might already have such a list of (legacy) data types somewhere? It'd be nice to have an enum for these, similar to how we have one for standard SQL now.

[tswast]

Standard SQL version: google.cloud.bigquery.enums.StandardSqlDataTypes